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We present Ge rib waveguide devices fabricated on a Ge-on-insulator (GeOI) wafer as a proof-of-concept Ge mid-infrared photonics platform. Numerical analysis revealed that the driving current for a given optical attenuation in a carrier-injection Ge waveguide device at a 1.95 μm wavelength can be approximately five times smaller than that in a Si device, enabling in-line carrier-injection Ge optical modulators based on free-carrier absorption. We prepared a GeOI wafer with a 2-μm-thick buried oxide layer (BOX) by wafer bonding. By using the GeOI wafer, we fabricated Ge rib waveguides. The Ge rib waveguides were transparent to 2 μm wavelengths and the propagation loss was found to be 1.4 dB/mm, which may have been caused by sidewall scattering. We achieved a negligible bend loss in the Ge rib waveguide, even with a 5 μm bend radius, owing to the strong optical confinement in the GeOI structure. We also formed a lateral p-i-n junction along the Ge rib waveguide to explore the capability of absorption modulation by carrier injection. By injecting current through the lateral p-i-n junction, we achieved optical intensity modulation in the 2 μm band based on the free-carrier absorption in Ge.
© 2016 Optical Society of America
1. Introduction
Mid-infrared (MIR) photonics is now attracting considerable attention for sensing and optical communication applications [1,2]. The MIR spectrum from 2 to 15 μm provides unique sensitivity and specificity for many important molecules, which can potentially serve as the basis for a broad collection of sensors [3]. Furthermore, there is even growing interest in exploiting the 2 μm band to increase the transmission capacity in future optical fiber communication systems owing to the development of low-loss photonic crystal fibers and fiber amplifiers [4–7].
However, conventional MIR technologies encounter difficulties when applied to these new applications because the devices are discrete, bulky, expensive, and power hungry [8]. To overcome these difficulties, photonic integration technology is effective by shrinking the device footprint. So far, many new platforms including Si membranes, Si-on-SiN, Si-on-Al2O3, and Ge-on-Si platforms [8–13] have been developed for MIR photonics integration, among which Ge-on-Si platform is rather promising for providing a widest useful MIR wavelength range and certain process ease. Recently, there have been a few studies achieving very-low-loss MIR light transmission in Ge-on-Si waveguide [14–16]. Moreover other Ge-on-Si devices like planar concave gratings, arrayed waveguide grating and thermo-optic phase shifter have also been demonstrated [16–18]. However, Ge-on-Si platforms have some intrinsic drawbacks. First, the absorption in the Si cladding limits the available wavelength range; second, the relatively small difference in the refractive index between Ge and Si impedes the scaling down of the photonic device footprint; in addition, a defective Ge/Si interface due to lattice mismatch may also degrade device performance. Therefore, an alternative structure is strongly required.
For MIR light transmission, Ge is almost an ideal material since it exhibits high transparency over nearly the entire MIR range [2]. In particular, for telecommunications in the 2 μm band, recent progress on lasing and light detection based on strained Ge and GeSn materials [19–22] has made it highly feasible to realize the monolithic integration of both active and passive photonic components using a Ge-based platform. Furthermore, Ge has many favorable optical properties in the MIR range, such as a high refractive index (Si 3.45, Ge 4.1 at 2μm) [23] and strong χ(3) nonlinearity (100 times that of Si) [24,25]. Furthermore, bulk Ge also has a 3 times larger thermo-optic effect than bulk Si. Ge is also reported to have much larger free-carrier absorption than Si. According to some studies [26,27], Ge is also expected to shows very strong free-carrier absorption. At around 2 μm wavelength, the free-carrier absorption by holes in Ge is expected to be 10 times greater than in Si, which could potentially lead to high-efficiency absorption modulation applications for MIR light [27].
To take full advantage of these attractive properties of Ge, we previously proposed a Ge MIR photonics platform comprising a high-quality Ge-on-insulator (GeOI) wafer [28]. A Ge layer of 100-nm-order thickness on a thick buried oxide (BOX) layer enabled strong two-dimensional optical confinement in the Ge layer, which is essential for the compact integration of photonic devices such as Si photonic devices.
In this work, we present a proof-of-concept Ge MIR integrated photonics platform with passive and active Ge photonic devices on the platform. Using the active Ge photonic device, we explored optical intensity modulation based on the free-carrier modulation. Figure 1 shows a schematic of the Ge rib waveguide on our GeOI wafer with a lateral p-i-n junction. First, we performed numerical analysis of a Ge variable optical attenuator (VOA) based on free-carrier injection through the lateral p-i-n junction. Then, we evaluated Ge rib passive waveguides fabricated on a GeOI wafer at a 2 μm wavelength. We achieved good light transmission in the Ge rib waveguides and almost negligible bending loss at sharp bends in the 2 μm band owing to the strong optical confinement in the GeOI structure. We also demonstrated optical intensity modulation based on the free-carrier absorption in Ge by using carrier injection through a lateral p-i-n junction.
Fig. 1 Schematic image of Ge rib waveguide with lateral p-i-n junction on GeOI wafer for carrier-injection optical intensity modulation.
2. Numerical analysis of a carrier-injection Ge modulator
To estimate the modulation efficiency of a carrier-injection optical Ge modulator based on free-carrier effects, we numerically analyzed a Ge rib waveguide with a lateral p-i-n junction by performing a technology computer-aided design simulation (TCAD Sentaurus) and finite-element optical mode analysis.
Figure 2(a) illustrates the simulated device structure and its doping profile. We designed a Ge rib waveguide with a core width of 600 nm, core thickness of 220 nm, and slab thickness of 50 nm. The intrinsic Ge layer had a constant doping profile with a boron doping concentration of 1 × 1016 cm−3. To form p + - and n + -doped regions, boron and phosphorus doping with a concentration of 1 × 1020 cm−3 were applied to the left and right sides of the Ge slab region, respectively. Using this structure, the mode profiles were calculated using the finite element method. Figure 2(b) shows the electric field intensity distribution of the fundamental TE-like mode of the Ge rib waveguide with SiO2 cladding at a 1.95 μm wavelength.
Fig. 2 (a) Structure of a Ge rib waveguide with a lateral p-i-n junction and (b) electric field intensity distribution of the fundamental TE-like mode (at 1.95 μm) in a Ge rib waveguide with SiO2 cladding.
In accordance with [27], we calculated the attenuation property in a 0.5-mm-long Ge rib waveguide device under a current injection condition, as shown in Fig. 3(a). Simulation results for a Si device operated at a 1.55 μm wavelength with the same structure and doping profile were also obtained for comparison. Note that the specific wavelengths were used for Si and Ge to achieve similar fundamental TE-like mode confinement in both devices. During the TCAD simulation, we took into consideration the Shockley–Read–Hall and Auger recombination for free carriers. The carrier lifetimes for electrons and holes in both Ge and Si were set to 3 ns and Auger parameters were set to those of the bulk values in [29] Since these parameters were very strongly dependent on the crystal quality and process condition, such setting reduce the uncertainty in comparing Ge-based and Si-based devices. It can be seen in Fig. 3(a) that the Ge device at 1.95 μm shows more than two times greater attenuation than the Si device at 1.55 μm because of the large free-carrier absorption in Ge at a wavelength of approximately 2 μm as discussed in [27]. If the Si device with the same structure operates at a 1.95μm wavelength, the attenuation property will become even worse because the optical confinement becomes weaker. Therefore, it is necessary to compare the Ge and Si device with similar structure and mode confinement at the same time. As a result, the required injection current for a given attenuation in the Ge device is five times smaller than that in the Si device. Therefore, the carrier-injection-based Ge device is promising as an efficient VOA. For 3 dB attenuation, the injection current required in the 0.5-mm-long Ge device is approximately 1.1 mA, which is comparable to or less than that for a phase shift of π in carrier-injection Si optical modulators. Hence, a carrier-injection Ge optical modulator based on free-carrier absorption is feasible with pre-emphasis driving. The direct intensity modulation through the absorption modulation makes the device structure simple as compared with a Mach–Zehnder interferometer configuration, and thus more suitable for large-scale integration. Figure 3(b) shows a comparison between the current-voltage curves of forward-biased p-i-n junctions of the Ge and Si devices. It is clear that the Ge device has a much lower turn-on voltage than the Si device, which is attributed to the narrow bandgap in Ge. Therefore, in principle, combined with a reduced injecting current, we can expect a significant power reduction in carrier-injection Ge devices. Therefore, it is strongly considered that Ge is very promising for intensity modulation in applications of MIR light.
Fig. 3 Comparison of (a) attenuation properties and (b) current-voltage characteristics of Ge and Si devices.
3. High quality Ge-on-insulator wafer fabrication
There have been previous studies of GeOI wafer fabrication for electronic devices, which usually use a thin BOX structure [30,31]. However, for photonic devices, a thick BOX structure is indispensable for achieving strong optical confinement. To fabricate a high quality GeOI wafer with a thick BOX layer required for our Ge MIR photonics platform, we bonded a Ge bulk wafer and a Si wafer. To split the bonded Ge wafer, we used Smart-CutTM technologies [28]. Figure 4(a) illustrates the process flow of GeOI wafer fabrication.
Fig. 4 (a) Process flow of Ge-on-insulator wafer fabrication, and (b) cross-sectional TEM image of the fabricated Ge-on-insulator wafer.
First, a SiO2 capping layer was deposited on a precleaned bulk Ge wafer by plasma-enhanced chemical vapor deposition (PECVD) for protection. After hydrogen (H) ion implantation on the Ge surface, the SiO2 capping layer was removed by wet etching. Then, a 5-nm-thick Al2O3 layer was deposited on the Ge surface by atomic layer deposition (ALD), which later served as half of the bonding interface layer. Meanwhile, the same 5-nm-thick Al2O3 deposition was also performed on a Si handle wafer, on which we thermally grew a 2-μm-thick SiO2 layer in advance as a BOX layer. Here, Al2O3 was used as bonding interface instead of the SiO2 interface in conventional SOI fabrication because the Al2O3 interface has a stronger bonding strength than SiO2 one at low temperature [32]. The thin Al2O3 interface was considered to have a small absorption for MIR light since which is transparent in the wavelength range from 300 nm up to approximately 5 μm [33]. The two wafers were then manually bonded together and annealed in vacuum at 400 °C to induce splitting of the bonded bulk Ge wafer. After splitting, we performed chemical mechanical polishing (CMP) to reduce the Ge surface roughness. Finally, the GeOI wafer was annealed in N2 ambient at 500 °C to improve the Ge crystal quality. Currently the available wavelength range is limited from 2 μm to 4 μm because SiO2 is transparent only till 4 µm, which the Ge-on-Si waveguide platform is transparent till 8 μm. Note that the SiO2 BOX layer can be replaced by chalcogenide glass [34] or the SiO2 BOX underneath the Ge waveguide can be partially removed [35] to avoid absorption by SiO2 at mid-infrared wavelengths. In this study, we still used SiO2 as a BOX material to fabricate our proof-of-concept platform.
Figure 4(b) shows a cross-sectional transmission electron microscope (TEM) image of the fabricated GeOI wafer with a Ge layer thickness of 400 nm. We characterized the GeOI quality by Hall measurement, which revealed that the Ge layer had a carrier density of lower than 1 × 1016 cm−3 and a hole mobility of more than 2000 cm2/Vs, comparable with those of the original bulk Ge wafer.
4. Ge rib passive waveguides on GeOI wafer
To study the transmission properties of Ge rib waveguides at MIR wavelengths, straight waveguides and bends were fabricated on the GeOI wafer by a conventional Si CMOS process. First, the GeOI wafer was precleaned and the top Ge layer was thinned to 300 nm by reactive ion etching (RIE) with CF4 gas. Then, 2-μm-wide waveguide patterns were formed by electron beam (EB) lithography and RIE dry etching with CF4 gas. We fabricated Ge rib waveguides with a core thickness of 300 nm and a slab thickness of 100 nm. Finally, a 300-nm-thick SiO2 layer was deposited on the sample for passivation.
Figure 5(a) shows a scanning electron microscope (SEM) image of a fabricated Ge straight rib waveguide. To characterize its transmission property, we used an amplified spontaneous emission (ASE) light source with a center wavelength of 1.95 μm. The input MIR light was coupled to the Ge waveguide through a lensed fiber, and then the output light was again coupled to another lensed fiber, which was connected to an optical spectrum analyzer (OSA) to monitor the power and spectrum. We prepared straight Ge waveguides with lengths ranging from 1.4 to 6.1 mm by cleaving, and the propagation loss of straight Ge waveguides was estimated by the cut-back method, as shown in Fig. 5(b). A waveguide propagation loss of approximately 1.4 dB/mm was obtained in the Ge rib waveguides, which was approximately 10 times larger than in the Ge-on-Si waveguide. Since the strong optical confinement on the GeOI makes the propagation loss more sensitive to the sidewall roughness, a major part of the propagation loss was resulted from scattering due to a large sidewall roughness caused by the un-optimized dry etching process [36], which can be confirmed by the SEM image of the Ge rib waveguide shown in Fig. 5(a). Figure 5(d) shows an atomic force microscope image of the Ge surface after the GeOI thinning process, exhibiting a relatively small root-mean-square (RMS) surface roughness of 0.4 nm at a 10 × 10 μm2 area. The free-carrier absorption due to the background doping in the Ge wafer is estimated to be less than approximately 0.4 dB/mm [27]. Thus, the propagation loss is dominated by the sidewall roughness scattering, which can be reduced by optimizing the EB lithography and dry etching process. We also studied the wavelength dependence of the propagation loss by comparing the spectra of the output light after propagating through the Ge waveguides and the direct output from the ASE light source. Figure 5(c) shows the wavelength dependence of the power transmitted through a 1.4-mm-long Ge waveguide. There is clearly no dependence of the propagation loss on the wavelength in the wavelength range from 1.9 to 2.0 μm, indicating that the optical absorption due to an indirect transition in Ge is negligible.
Fig. 5 (a) Scanning electron microscope image of a Ge rib waveguide, (b) propagation loss of Ge rib waveguides, (c) transmitted spectrum of a 1.4-mm-long Ge rib waveguide and (d) AFM image of Ge surface after thinning.
The transmission property in a Ge rib waveguide with bends was investigated on the basis of the result for straight waveguides. We fabricated Ge waveguides with bends having radii from 5 to 30 μm; a SEM image of a series of 5-μm-radius bends is shown in Fig. 6(a). Using the same method as for straight-waveguide measurement, we obtained the output power of Ge waveguides with bends of various radii. After compensating for the propagation loss in the straight waveguide part, the bend loss per 90° turn was obtained as a function of the bend radius, as shown in Fig. 6(b). A bend loss of approximately −0.2 dB at each turn was obtained in the Ge bend waveguide. We could estimate a measurement error of about 0.1 dB for bends at each turn, which could be further improved by increasing the number of bends. Furthermore, it is clear that the bend loss showed no dependence on the radius for radii from 5 to 30 μm, which suggest that a strong optical confinement was achieved in the Ge rib waveguide owing to the GeOI structure.
Fig. 6 (a) Scanning electron microscope image of 5-μm-radius Ge bends, and (b) bend loss of Ge rib waveguide as a function of bend radius.
5. Optical intensity modulation by carrier injection into Ge rib waveguide
As a key element in MIR integrated photonics, optical modulation plays an important role in light manipulation. To demonstrate the capability of the optical intensity modulation based on free-carrier absorption in Ge, we fabricated a Ge rib waveguide with a lateral p-i-n junction as a VOA. Figure 7 illustrates the fabrication process flow of the Ge MIR VOA. Firstly, the Ge rib waveguide pattern was formed by EB lithography and dry etching process as mentioned before. Then, phosphorus (P) and boron (B) ion implantation were carried out to form n + and p + regions for p-i-n junction formation, respectively, which was followed by annealing in nitrogen ambient at 600 °C for dopant activation. After passivating the device surface with a 300-nm-thick PECVD SiO2 layer, via holes were formed by wet etching. Finally, 60-nm-thick nickel electrodes were deposited by thermal evaporation.
Figure 8(a) shows a SEM image of the fabricated Ge MIR VOA. To characterize the intensity modulation property, we measured optical attenuation of the Ge VOA in the 2 μm band by injecting current through the lateral p-i-n junction. Figure 8(b) shows the measured optical attenuation of a 250-μm-long Ge MIR VOA. Owing to the free-carrier absorption in Ge, an increase in the optical attenuation was observed when we increased the injection current.
Fig. 8 (a) Scanning electron microscope image of a Ge variable optical attenuator and (b) attenuation characteristics of the Ge variable optical attenuator in the 2 μm band as a function of injection current.
However, the attenuation efficiency in the Ge VOA was lower than that expected. To clarify the reason for this, we investigated the electrical characteristics of the lateral p-i-n junction in the Ge rib waveguide. Figure 9(a) shows the current-voltage curve of a Ge p-i-n junction with a junction length of 50 μm. A typical ideality factor of approximately 1.7 was estimated for the fabricated p-i-n junctions, suggesting that a deficient junction may limit free-carrier injection into the Ge channel. In fact, the difficulty in performing effective n-type doping into Ge by ion implantation process may result in poor p-i-n junction formation [37]. To further clarify the junction properties, we used the transmission line method (TLM) to evaluate the resistivity of the n + - and p + -doped regions. Figure 9(b) shows the carrier concentrations in the doped Ge layers estimated from the measured resistivities; the inset illustrates the schematic structure of the TLM pattern. The estimated hole concentration was 3 × 1018 cm−3, while the carrier concentration of 3 × 1017 cm−3 in the n + - (P-doped) Ge region was significantly lower than the initial doping concentration expected from the implanted dose, suggesting a problem in the process of n-type doping, that degraded the attenuation property in the Ge MIR VOA. We consider that the high diffusivity of implanted phosphorus during activation annealing may have prevented us from forming high-quality junction [38], so that optimization of the annealing temperature and duration would be helpful to improve the junction quality. Moreover, advanced ultrashort annealing technique such as flash lamp annealing or laser annealing can also improve activation in the n + Ge region [39]. Furthermore, we could try spin-on-glass method to form the heavily doped region, which is also very promising to realize a good p-i-n junction on the GeOI substrate.
Fig. 9 (a) Typical current-voltage curve of fabricated Ge p-i-n junction and (b) carrier concentrations in n+/p + - doped Ge layers estimated by transmission line method.
Although the device performance was lower than expected, we successfully achieved the absorption modulation in the 2 μm wavelength band in a Ge rib waveguide on a GeOI wafer by carrier injection for the first time. Upon an optimization of the p-i-n junction formation process, we believe that the modulation performance can be greatly improved in the future.
6. Conclusion
We have successfully demonstrated a proof-of-concept Ge MIR integrated photonics platform by developing Ge MIR photonic components including Ge rib passive waveguides and a carrier-injection VOA on a GeOI wafer. Since a strong free-carrier effect is expected in Ge at MIR wavelengths, a carrier-injection Ge device has great potential for optical intensity modulation in the MIR. To realize a carrier-injection device based on a Ge rib waveguide operating at MIR wavelengths, we fabricated Ge rib waveguides on a GeOI wafer and characterized their transmission properties. Optical transmission in the 2 μm band with 1.4 dB/mm propagation loss was achieved in the Ge rib waveguides. Owing to the strong optical confinement, we also achieved negligible bend loss of −0.2 dB per 90° turn in Ge bend waveguides with 5 μm radius. By using the Ge rib waveguides with lateral p-i-n junctions, we demonstrated optical intensity modulation based on the free-carrier absorption in Ge by injecting carriers through the Ge p-i-n junction for the first time. Currently, the propagation loss of the Ge rib waveguide is higher than that of Ge-on-Si waveguides. However, there is plenty of room for improving the device performances on the GeOI platform through process optimization. Thus, we demonstrate that the Ge MIR photonics platform on the GeOI substrate is very promising for MIR integrated photonics.
Acknowledgments
This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) “Integrated Photonics-Electronics Convergence System Technology (PECST)” project and a MEXT Grant-in-Aid for Scientific Research (S).
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